HA
Turns out the double colon ":" in C:\Windows\Fonts etc was acting as a split so when i was inputting the font's full path ffmpeg was reading my command as follows
original command
" -vf drawtext=fontfile='C:\\Windows\\fonts\\arial.ttf'|text='test' "
ffmpeg's interpretation
-vf drawtext= # command
fontfile='C # C is the font file because the : comes after it signalling the next key
arial.ttf' # is the next key after fontfile = C (because the C is followed by a : signalling the next key)
:text # is the value the key "arial.tff" is pointing to
='test' # is some arb piece of information put in by that silly user
So to fix it you need to elinate the : in the font file path.
My final working code:
import subprocess
ffmpeg = "C:\\ffmpeg_10_6_11.exe"
inVid = "C:\\test_in.avi"
outVid = "C:\\test_out.avi"
subprocess.Popen(ffmpeg + " -i " + inVid + ''' -vf drawtext=fontfile=/Windows/Fonts/arial.ttf:text=test ''' + outVid , shell=True)